1. Field of the Invention
This invention relates generally to implantable devices, such as cardiac stimulators, designed to be situated within a living body and to exchange information with devices located outside the body. More particularly, the invention relates to a novel technique for processing and exchanging data between an implantable device and a remote device which makes efficient use of power and signal processing capabilities within the implantable device.
2. Description of the Related Art
In recent years increasingly sophisticated systems have been developed for monitoring and controlling certain physiological processes via implanted devices. Such devices are typically placed within a patient's body and remain resident within the patient's body over extended periods of time. One such device, commonly referred to as a cardiac stimulator, is commonly implanted in a patient's chest region and includes circuitry both for monitoring the functioning of the patient's heart as well as for providing stimulus for the heart when needed.
Conventional implantable cardiac stimulators include one or more electrical leads which extend between electronic circuitry provided within the device housing and portions of the patient's heart. For example, leads extending from the stimulator may be terminated in the right atrium and right ventricle of the patients heart to provide both sensing and stimulation capabilities. The circuitry is programmed to execute desired functions, such as monitoring, stimulating, and storing of diagnostic or other data. A power supply is implanted with the device to furnish the electrical energy needed for its operation.
Through their relatively short history, cardiac stimulators and other implantable devices have experienced very considerable evolution. For example, early cardiac stimulators provided fixed rate stimulating pulses designed to regulate the patient's heart beat only. Later designs, sometimes referred to as "demand" pacemakers, also offered heart monitoring capabilities, providing stimulating pulses only as needed based upon the monitored functioning of the heart. Further improvements in cardiac stimulators included programmable rate pacemakers, dual chamber pacemakers, and "rate-responsive" pacemakers, each providing increased flexibility and adaptability of the monitoring and stimulation functions to more closely conform to the needs and physiological parameters of the patient, such as the patient's level of physical activity.
Throughout the evolution of cardiac stimulators and other implantable devices, a persistent problem has been the efficient provision and use of electrical energy. In general, the power source, typically including a specially designed electric battery, is implanted with the electronic circuitry to provide all power necessary for the monitoring, stimulation, programming and other functions of the implantable device over extended periods of time, often measured in years or decades. To provide the longest possible life to the implanted power source, therefore, it is generally a goal in the design of such devices to reduce the power needed for all aspects of their function. For example, the replacement of early fixed rate pacemakers with demand pacemakers significantly reduced the energy continuously dispensed by the device by generating stimulating signals only as needed, thereby prolonging the effective life of the power source. Other developments have also extended the useful life of such power sources, although further improvements are still needed.
A particularly useful function of implantable devices involves the ability to transmit and to receive information between the implantable device and an outside programming or monitoring unit. Data exchange between the implantable device and the external unit permits parameters, such as physiological data, operational data, diagnostic data, and so forth, to be transmitted from the implantable device to a receiver from which the data can be accessed and further processed for use by an attending physician. The data is particularly useful for gaining insight into the operation of the implantable device as well as the state of the patient's organs and tissues. The ability to exchange data in this manner also permits the physician to reprogram or reconfigure the implantable device as may be required from time to time due to evolution of the patient's condition.
Data exchange between an implantable device and a remote, outside device is often accomplished by "waveform telemetry" in which the data is conveyed through the patient's tissue and skin. Early waveform telemetry systems employed in implantable cardiac stimulators transmitted signals through analog encoding. For example, in one known type of pacemaker, analog samples representing operational or physiological parameters are transmitted as the pulse position of a radio-frequency pulse train. The pulse train is output by either the implantable device or the outside device, and is interpreted or decoded upon receipt by the other device. While such techniques are extremely useful for gaining access to information relating the performance of the patient's organs and of the implantable device, analog telemetry circuits typically yield low resolution and often AC-coupled and uncalibrated signals, effectively limiting their utility and reliability.
To address the shortcomings of analog telemetry systems, digital telemetry schemes have been developed. For example, certain digital telemetry systems are presently in use wherein a radio-frequency carrier or radio-frequency pulse train is modulated by digital information corresponding to samples of the analog signals to be telemetered. Such digital data communication methods make use of an analog-to-digital (A/D) converter for transforming samples of analog signals into digital format for transmission. If multiple analog signals are to be transmitted, an analog signal multiplexer is employed to select one signal at a time to feed to the A/D converter. A programmer or a telemetry system controller selects the channel from which the next sample is to be converted prior to transmission. However, such processing reduces the sampling rate per signal due to the relatively large portion of time and telemetry channel bandwidth which must be used for communicating the channel information. Moreover, a relatively fast A/D converter is required because the telemetry system must wait for the conversion to be completed before being able to transmit the data. The use of a fast A/D converter results in considerable energy usage, reducing the effective life of the implantable power source.
Alternatively, a predetermined data acquisition sequence may be established to eliminate the need for continuously communicating the channel to be converted. This alternative, however, limits the flexibility of the system as the number and identity of channels to be transmitted generally cannot be changed without first reconfiguring the sequencer. Moreover, this technique requires the sampling process to be synchronized with read operations executed by the telemetry circuit, as asynchronous operation may yield transmission or reception of invalid or misinterpreted data.
There is a need, therefore, for an improved technique for exchanging data between an implantable device and a device external to a patient's body. There is a particular need for a telemetry technique which is capable of transmitting digitized data to and from an implantable device, but which avoids certain of the drawbacks of existing systems as summarized above.